Data line receivers inherently exhibit high impedance. Data line drivers, however, present a load on a communications line. As a consequence, a communications line serving several data line drivers may become unduly loaded and develop transmission problems. Electrically isolating inactive drivers can reduce the loading on the communications line.
Optocouplers are one class of devices used to transmit signal information between respective circuitry while maintaining electrical isolation therebetween. This is accomplished since optical signals, and not electrical signals, are passed between the light emitters and the photodetectors of an optocoupler, which are connected to respective portions of the circuitry. The size, cost and performance of optocouplers makes these devices ideal to implement isolation for data communication lines and interfaces.
However, optocouplers are usually single-ended or unbalanced, and have just two states: high and low. In contrast, noise-insensitive data formats are typically differential, offering a balanced signal with respect to a common reference. Further, to provide additional states beyond simply high and low, more devices are needed, i.e., one device for every two states.
It is therefore desirable to have a single channel optical interface device that will support differentially-formatted data and offer tri-state line termination characteristics. It is also desirable to provide an optical interface device having transfer characteristics that are independent of time, temperature, and variations in the efficiency of the optical source.